title: Metabolism of irinotecan (CPT-11) by human hepatic microsomes: participation of cytochrome P-450 3A and drug interactions.
abstract: Irinotecan (CPT-11) is a water-soluble analogue of camptothecin showing activity in colon cancer. Recently, we identified a major metabolite of CPT-11 in patients' plasma, 7-ethyl-10-[4-N-(5-aminopentanoic acid)-1-piperidino] carbonyloxycamptothecin (APC), which is produced by the oxidation of the distal piperidine ring (P. Rivory et al, Cancer Res., 56: 3689-3694, 1996). As with all active camptothecin derivatives, CPT-11 is subject to spontaneous interconversion between a lactone and a carboxylate form in aqueous media. The kinetics of biotransformation of the two forms of CPT-11 into APC was studied using pooled human liver microsomes. The formation of APC was characterized by the following parameters: Km = 18.4 +/- 1.4 and 39.7 +/- 11.6 microM; and Vmax = 26.0 +/- 0.6 and 13.4 +/- 1.7 pmol/min/mg protein for the lactone and carboxylate forms of CPT-11, respectively. This reaction was found to be catalyzed principally by cytochrome P-450 (CYP) 3A because of three key results: (a) the CYP 3A-selective inhibitors ketoconazole (1 microM) and troleandomycin (100 microM) inhibited APC formation by 98 and 100%, respectively, mostly in a competitive way; (b) using microsomes from transfected lymphoblastoid cells expressing specific CYPs, we found that only those from CYP 3A4 cDNA-transfected cells transformed CPT-11 into APC; and (c) using 15 individual preparations of human liver microsomes, we observed highly significant correlations between the activity of CPT-11 metabolism into APC and both immunoreactivity with anti-CYP 3A antibodies and testosterone 6beta hydroxylation, an activity specifically mediated by CYP 3A. The effect on this metabolism of 11 drugs used at 100 microM was studied with CPT-11 lactone at 25 microM. Amikacin, Bactrim, ciprofloxacin, rocephine, 5-fluorouracil, metoclopramide, morphine, and paracetamol had no effect, but ondansetron, loperamide, and racecadotril inhibited this pathway by 25, 50, and 50%, respectively. These concentrations exceed those expected in vivo. APC formation in patients may thus be influenced by coadministered ketoconazole therapy and may decline after administration of CPT-11 because of the lactonolysis of the latter.
authors.affiliation: Institut Bergonié and Université Victor Segalen Bordeaux 2, France.
authors.forename: M C
authors.initials: MC
authors.lastname: Haaz
authors.forename: L
authors.initials: L
authors.lastname: Rivory
authors.forename: C
authors.initials: C
authors.lastname: Riché
authors.forename: L
authors.initials: L
authors.lastname: Vernillet
authors.forename: J
authors.initials: J
authors.lastname: Robert
mesh_terms: D000972:Antineoplastic Agents, Phytogenic / Q000378:metabolism* / Q000494:pharmacology; D001189:Aryl Hydrocarbon Hydroxylases; D001711:Biotransformation / Q000187:drug effects; D002166:Camptothecin / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D051544:Cytochrome P-450 CYP3A; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics / Q000378:metabolism*; D004347:Drug Interactions; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D000077146:Irinotecan; D007654:Ketoconazole / Q000494:pharmacology; D008862:Microsomes, Liver / Q000378:metabolism*; D015394:Molecular Structure; D010089:Oxidoreductases, N-Demethylating / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D013329:Structure-Activity Relationship; D014162:Transfection; D014217:Troleandomycin / Q000494:pharmacology
chemical_list: D000972:Antineoplastic Agents, Phytogenic; D065607:Cytochrome P-450 Enzyme Inhibitors; D004791:Enzyme Inhibitors; C100720:RPR 121056; D011994:Recombinant Proteins; D000077146:Irinotecan; D003577:Cytochrome P-450 Enzyme System; D014217:Troleandomycin; D001189:Aryl Hydrocarbon Hydroxylases; D051544:Cytochrome P-450 CYP3A; D010089:Oxidoreductases, N-Demethylating; D007654:Ketoconazole; D002166:Camptothecin